1. Technical Field of the Invention
The present invention relates generally to digital color image sensors, and specifically to image processing of sensor values.
2. Description of Related Art
Electronic image sensors are predominately of two types: CCDs (Charge Coupled Devices) and CMOS-APS (Complimentary Metal Oxide Semiconductor-Active Photo-detector Sensors). Both types of sensors typically contain an array of photo-detectors (e.g., pixels), arranged in rows and columns or arranged in another pattern, that sample color within an image. Each photo-detector measures the intensity of light within one or more ranges of wavelengths, corresponding to one or more perceived colors.
In addition, both types of sensors may include a color filter array (CFA), such as the CFA described in U.S. Pat. No. 3,971,065 to Bayer (hereinafter referred to as Bayer), which is hereby incorporated by reference. With the Bayer CFA, each photo-detector sees only one wavelength range, corresponding to the color red, green or blue. To obtain the sensor values for all three primary colors at a single photo-detector location, it is necessary to interpolate the color sensor values from adjacent or surrounding photo-detectors. This process of interpolation is called demosaicing.
Unless the demosaiced image is immediately output, the demosaiced image must be stored in a storage medium for later retrieval. Due to the large amount of memory necessary for storing demosaiced images, many applications utilize image compression techniques that represent the demosaiced images with less data in order to save storage costs or reduce transmission time. Compression can be performed using a lossless compression technique or using a lossy technique, such as JPEG (Joint Photographic Experts Group). A discussion of the JPEG technique can be found in: W. Pennebaker and J. Mitchell, “JPEG: Still Image Data Compression Standard,” New York: Van Nostrand Reinhold, 1993, which is hereby incorporated by reference.
To improve the image quality, many applications convert the photo-detector values to another color space prior to compressing the image. For example, an array of photo-detectors arranged in rows and columns and fitted with a Bayer CFA typically has the following repeat unit:
                    G                    R                            B                    G              ,which is stepped and repeated throughout the array of photo-detectors. Thus, each repeat unit produces four photo-detector values: two green, one red and one blue. After demosaicing, the repeat unit has twelve photo-detector values: four green, four red and four blue. Since the human eye is more sensitive to changes in luminance, which is dominated by green, than chrominance, which is dominated by red and blue, the twelve demosaiced photo-detector values are typically converted into the YCbCr color space, followed by a 4:1:1 decimation. The result is four luminance values (Y) and two chrominance values (Cr, which corresponds to red chrominance, and Cb, which corresponds to blue chrominance) for each block of four photo-detectors. To convert the twelve demosaiced color values into the four luminance and two chrominance values, one of a number of well-known conversion matrices may be used.
One of the drawbacks of the above standard image processing techniques is that the substantial processing required to perform demosaicing and color space transformation increases the cost and complexity of conventional image processing systems. Another drawback of the above standard image processing techniques is that, due to area constraints on image sensors, the extensive image processing (demosaicing, color space transformation and image compression) is performed by an image processing system separate from the image sensor. Therefore, what is needed is a digital image sensor capable of combining both sensing and at least part of the image processing to reduce the cost and complexity of image processing.